The main physical characteristics needed to evaluate the degree to which a subterranean earth formation will produce a desired liquid such as water or oil are its porosity, liquid saturation, permeable bed thickness, and permeability. Permeability is a measure of the ease with which a formation permits a fluid of given viscosity to flow therethrough. To be permeable, a formation must have interconnected porosity (pores, vugs, capillaries, or fractures). Porosity which is not interconnected fails to contribute to formation permeability. The permeability of a given kind of earth formation to the flow of any homogeneous fluid is constant with time provided the fluid does not interact with the rock or other material comprising the formation.
Because of the importance of the permeability characteristic of a subterranean earth formation, several techniques have been used to measure the permeability of such formations, especially those surrounding wellbores. In one common procedure, as the wellbore is drilled, core samples of the formations are extracted to the earth's surface and then are transported to the laboratory for permeability analysis. Commonly, during such analysis the sample is placed in a tight-fitting container, and the flow rate through the sample of a fluid having a known viscosity and under a known pressure is measured. Laboratory analysis of core samples is time consuming, costly and cumbersome. Moreover, the samples are sometimes altered or damaged during extraction, transport or preparation for analysis, which introduces error in the analysis.
A Spontaneous-Potential ("SP") technique has been used to detect the location and thickness of relatively permeable earth formations. The SP technique involves a recording versus wellbore depth of the difference between the potential of a movable electrode in the wellbore and the fixed potential of a surface electrode. Variances or deflections of the SP recording curve result from electric currents flowing in the mud in the wellbore which are caused by electromotive forces in the earth formations, which forces are of electrochemical and electrokinetic origins. The SP technique detects primarily the boundaries of relatively permeable earth formations, and there is no direct relationship between the value of permeability and the magnitude of SP recording curve deflections. Moreover, the SP technique is extremely adversely affected by a variety of common wellbore and earth formation conditions such as are described in chapter two of the 1972 edition of Schlumberger Limited's "Log Interpretation Volume I--Principles."
Microresistivity devices are used to measure the resistivity of flushed earth formations and to delineate permeable formations by detecting the presence of mud cake along the wellbore wall. However, such measurements generally cannot provide accurate inferences of the formation permeability.
Many other logging techniques such as induction logging and neutron logging have been used to detect a variety of subterranean earth formation characteristics, but none has been used to determine the permeability or relative permeability of such formation.
A patentability search was conducted for the present invention, and the following patents were uncovered:
______________________________________ U.S. Pat. No. Inventor Issue Date ______________________________________ 3,158,023 Brillant November 24, 1964 3,463,230 Dodson August 26, 1969 3,550,445 Kiel December 29, 1970 3,559,476 Chiang-Hai Kuo et al. February 2, 1971 3,636,762 Kuo et al. January 25, 1972 3,604,256 Prats September 14, 1971 3,871,218 Louis March 18, 1975 ______________________________________
U.S. Pat. No. 3,158,023 estimates the permeability of ground around a borehole by measuring the rate at which water must be pumped from the borehole in order to maintain a predetermined lowering of the water level therein.
U.S. Pat. No. 3,463,230 disclosed a method of performing a relative permeability survey of the earth surrounding an oil well which includes plugging an increment of the earth by the introduction of floatable particles within the well and then making an injection test of the unplugged portion.
U.S. Pat. No. 3,550,445 discloses a method for testing wells for the existence of permeability damage to an earth formation.
U.S. Pat. No. 3,559,476 teaches a method for measuring the reservoir property of a porous earth formation surrounding a well by establishing a pulsating flow of fluid through the well adjacent to the porous earth formation into and out of the formation at rates that vary with time in accordance with a predetermined periodic function. The variations with time of the pressure in the well are measured and the phase shift and amplitude of the pressure variations with time relative to the variations with time of the rates of the pulsating flow of fluid are determined.
U.S. Pat. No. 3,636,762 relates to a method of measuring various reservoir properties of wells such as the skin factor and the permeability thickness product, which method comprises the steps of rapidly increasing the rate of fluid injection into a porous earth formation penetrated by a well, maintaining the fluid injection rate constant at a high rate, and recording the variation with time of the fluid injection pressure.
U.S. Pat. No. 3,604,256 relates to a method for measuring the average vertical permeability of a subterranean earth formation near a wellbore comprising the steps of sealing off the wellbore, perforating the wellbore at two vertically spaced locations, sealing the wellbore between the perforations, injecting fluid at a substantially constant rate through one of the perforations, and measuring the pressure response in the wellbore at the other perforation.
U.S. Pat. No. 3,871,218 discloses a method of determining the permeability characteristics of a medium surrounding a borehole comprising the steps of dividing the borehole longitudinally into three adjacent cavities, producing a flow of liquid in each cavity and in the corresponding regions of the medium, measuring the flow rate of liquid flowing in the intermediate cavity, and measuring the liquid pressure in the intermediate cavity and in the corresponding region of the medium at known distances from the borehole axis, and determining the permeability characteristics from the flow rate and the liquid pressure measurements.